A memory cell in an integrated circuit, such as a dynamic random access memory (DRAM) array, typically comprises a charge storage capacitor (or cell capacitor) coupled to an access device such as a metal oxide semiconductor field effect transistor (MOSFET). The MOSFET functions to apply or remove charge on the capacitor, thus effecting a logical state defined by the stored charge. The amount of charge stored on the capacitor is proportional to the capacitance C, defined by C=kk0A/d, where k is the dielectric constant of the capacitor dielectric, k0 is the vacuum permittivity, A is the electrode surface area and d is the distance between electrodes.
FIG. 1 illustrates a portion of a conventional DRAM memory circuit containing two neighboring DRAM cells 10. For each cell, one side of the storage capacitor 14 is connected to a reference voltage Vr, which is typically one half of the internal operating voltage (the voltage typically corresponding to a logical “1” value) of the circuit. The other side of the storage capacitor 14 is connected to the drain of an access field effect transistor 12. The gate of the access field effect transistor 12 is connected to a word line 18. The source of the field effect transistor 12 is connected to a bit line 16. With the cell 10 connected in this manner, it is apparent that the word line 18 controls access to the storage capacitor 14 by allowing or preventing a signal (corresponding to a logic “0” or a logic “1”) on the bit line 16 to be written to or read from the storage capacitor 14 only when a signal from the word line 18 is applied to a gate of the access transistor 12.
Capacitors, like the capacitors 14 shown in FIG. 1, suffer from current loss in two ways: (1) direct current leakage loss, which results in high power consumption, and (2) dielectric relaxation. Direct current leakage loss accounts for charge transport from one electrode to another across the dielectric. Direct current leakage also creates the need for a DRAM cell to be refreshed at frequent periods, as charge stored in the capacitor leaks to adjacent active areas on the memory cell. Dielectric relaxation, on the other hand, is a phenomenon that refers to a residual polarization within a dielectric material of a memory storage device when a voltage is applied to the device. Dielectric relaxation, which can be described mathematically in accordance with the Curie-von Schweidler behavior formula, is time-dependent. At least in ideal operation, however, dielectric relaxation is independent of the electrode material, dielectric thickness, and any direct leakage current from the dielectric layer. More significantly, dielectric relaxation is dependent on the type of dielectric materials used and becomes increasingly worse for high-k dielectric materials, which for other reasons are increasingly favored in integrated circuit fabrication.
FIG. 2A is an illustrative graph of current stored in a capacitor versus time for a capacitor 14 (FIG. 1). As shown in FIG. 2A, a capacitor that suffers only from direct current leakage, has a nearly horizontal slope (line 5) on this graph, meaning that the direct current loss is not dependent on time. As shown by line 1, however, when dielectric relaxation losses are realized, the slope of the line 1 changes significantly, and thus, the current loss from dielectric relaxation is dependent on time. Each of the dotted sloped lines 2-4 show possible relaxation leakage from a capacitor, which may change based on other factors such as the temperature and the applied voltage.
The current losses from dielectric relaxation are undesirable for many reasons. In DRAM devices, for example, dielectric relaxation can affect the effectiveness of some dielectrics, such as high-k dielectrics, used in the DRAM storage capacitors. In addition, dielectric relaxation can create a threshold shift that severely deteriorates MOSFET performance.
There is a need, therefore, for a memory cell capacitor structure that does not suffer from the undesirable effects of dielectric relaxation but rather can use this phenomenon in a beneficial manner. Accordingly, there is also needed a simple method of producing and operating the desired capacitor structure.